There has been known a hydraulic drive system for a work machine having a regeneration circuit by which hydraulic fluid discharged from a boom cylinder due to falling of a boom by its own weight is regenerated for an arm cylinder, and an example thereof is described in Patent Document 1.
The hydraulic drive system for a work machine described in Patent Document 1 has a control unit by which delivery flow rate of a hydraulic pump is reduced when hydraulic fluid discharged from a boom cylinder is regenerated for an arm cylinder, and engine speed is lowered in the case where delivery flow rate of the hydraulic pump at the time of a combined operation is not more than a prescribed flow rate.